More likely (given other references in the wikipedia article to "non-resident males") there is a discrepancy between who gets counted as a resident by the count

More likely (given other references in the wikipedia article to "non-resident males") there is a discrepancy between who gets counted as a resident by the county and who can be registered there by the state. Additionally, Texas might or might not have a habit of purging dead or emigrant voters from their voting rolls, which would skew the number of registered voters. Lastly, yes, the census is almost certainly an undercount, including in the more (shall we say) government-counting-everybody-skeptical rural regions of the country.